The importance of semiconductor lasers is rapidly increasing along with progress in the opto-electronic field, such as optical fiber communication, optical data processing, storage and solid state laser pumping. High power 808 nm laser diodes for Nd:YAG laser pumping and other universal laser devices appears to be the future generation of blue and other visible light lasers in information processing and fiber optic telecommunications.
Buried-ridge lasers have the advantages of providing excellent optical and electrical confinement for carriers. In addition, these lasers provide low threshold current density, stable guide modes, and near-circular far-field pattern. In particular, such structures can couple the laser light output efficiently into an optical fiber. The internal waveguide can be formed by etching the semiconductor material and regrowing a lower index material around the lasing stripe.
The technique of LP-MOCVD is well adapted to the growth of the entire composition range of GaInAsP layers of uniform thickness and composition that is lattice matched to GaAs over areas of more than 10 cm.sup.2. This results first from the ability of the process to produce abrupt composition changes and second from the result that the composition and growth rate are generally temperature independent. It is a versatile technique, numerous starting compounds can be used, and growth is controlled by fully independent parameters.
Growth by MOCVD takes place far from a thermodynamic equilibrium, and growth rates are determined generally by the arrival rate of material at the growing surface rather than by temperature-dependent reactions between the gas and solid phases. In contrast to LPE growth, it has been found that during MOCVD growth of a double heterostructure, GaAs can be grown directly on GaInAsP with no disturbance of the active layer, i.e., there is no effect equivalent to melt-back.
One of the key reasons for the usefulness of the MOCVD method is the possibility of obtaining high-purity and therefore high-mobility Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y heterostructures. As long-wavelength 1.0-1.65 .mu.m GaInAsP electro-optical devices become more widely used, motivated by low fiber absorption and dispersion, high transmission through water and smoke, and greatly enhanced eye safety at wavelengths greater than 1.4 .mu.m, LP-MOCVD offers the advantages of smooth uniform surfaces, sharp interfaces (lower than 5 .ANG. for GaInAsP/GaAs), uniformly lower background doping density, and economy of scale for large-area devices.
Recent studies have shown the feasibility of using InGaAsP/GaAs heterostructures as diode lasers. The diodes can be used successfully for solid state laser pumping and can be interchanged with lasers based on AlGaAs/GaAs heterostructures.
Laser diodes emitting at 808 n wavelength are important sources for the pumping of YAG:Nd lasers. AlGaAs/GaAs lasers are commonly used for this purpose, but there are several problems with these structures: oxidation of AlGaAs layers which makes further regrowth and device fabrication difficult; higher growth temperature which may not be compatible with monolithic integration; and the presence of dark line defects and dislocation migration which can cause degradation in performance. Most of these problems can be attributed to the presence of Aluminum. High-power quantum well lasers based on liquid phase epitaxy (LPE) grown GaInAsP/GaAs structures do demonstrate characteristics competitive to the best existing AlGaAs/GaAs separate confinement heterostructure-single quantum well (SCH-SQW) lasers, but, as stated above, growth by LPE presents several major disadvantages.